Please comment on maturandum and periclitandum. Participle? Gerund? Gerundive? How do they fit into the sentence construction?

Below is my literal translation. This is not idomatic English, but hints in English of what the Latin grammar might be. I'm unconcerned with finding a good Englishing, only with understanding the Latin grammar.

Caesar decided that it-must-be-speeded up by him, if it must, concerning the completion of the bridge, be-risked, in order that before more troops were concentrated in that place he might fight a decisive battle.

I'm reading Caesar as a sideline to studying Floyd and Rita. I'm having trouble finding a helpful explanation of the future passive participle/gerund/gerundive construction as used in Caesar.

Gerunds and gerundives can be confusing because different books use different terminology.... But this is what seems to be most common (e.g. in Wheelock):

1) Gerund: a verbal noun. It only has forms in the oblique cases (for nominative, you use an infinitive instead). So, for a verb like porto, the gerund is portandi, portando, portandum, portando. "To err is human": errare est humanum (infinitive). "Love of reading": amor legendi."We become happier by living well": Feliciores fimus vivendo bene.

2) Gerundive: a verbal adjective. It has all the inflectional forms: portandus, -a, um. Whenever a gerund would take a direct object, Latin normally uses a gerundive agreeing with the noun instead. You could say, Discimus legendo libros, "We learn by reading books," but it's more usual to say, Discimus libris legendis. (Some might say that this "literally" means "We learn by books being read," but I personally don't find it helpful to remember a "literal" meaning that is almost nonsensical in English, and has a completely different nuance anyway.) So this explains how perficiendis is being used in your Caesar sentence: in perficiendis pontibus, "in completing the bridges." It's equivalent to in perficiendo pontes, but you would rarely say that in Latin.

3) The future passive participle is just another name for the gerundive. One of its most common uses is to express obligation: hic liber mihi legendus est. "This book must be read by me," i.e. (in real English) "I have to read this book." The subject of the sentence is the thing that must be done, and the gerundive agrees with it. If the verb is intransitive, the gerundive is neuter singular (impersonal). The person by whom the thing must be done is dative. Caesari maturandum est. "Caesar must hurry." The sentence in your passage is a form of this, but it's in indirect discourse (with censuit), and esse is omitted (as it frequently is in such situations). Caesar maturandum sibi censuit, "Caesar decided that he had to hurry." [Caesari] in pontibus perficiendis periclitandum est. "Caesar must run a risk in completing the bridges."

So the whole thing put together means, "Caesar decided that he had to hurry, if it was necessary to run the risk in completing the bridges, in order to fight a decisive battle before more troops were gathered there." (But this translation doesn't really show that the ut ... dimicaret clause goes closely with maturandum: i.e. he decided that, if he was going to try to complete the bridges, he had to hurry in order to fight a decisive battle before more enemy troops arrived.)

Here's the translation by W. A. McDevitte and W. S. Bohn which is all over the Net // Ecce traductio in anglicum ubiquè in interrete.

http://classics.mit.edu/Caesar/gallic.7.7.html wrote:Caesar on being informed of these movements was of opinion that he ought to make haste, even if he should run some risk in completing the bridges, in order that he might engage before greater forces of the enemy should be collected in that place. For no one even then considered it an absolutely necessary act, that changing his design he should direct his march into the Province, both because the infamy and disgrace of the thing, and the intervening mount Cevennes, and the difficulty of the roads prevented him; and especially because he had serious apprehensions for the safety of Labienus whom he had detached, and those legions whom he had sent with him. Therefore, having made very long marches by day and night, he came to the river Loire, contrary to the expectation of all; and having by means of the cavalry, found out a ford, suitable enough considering the emergency, of such depth that their arms and shoulders could be above water for supporting their accoutrements, he dispersed his cavalry in such a manner as to break the force of the current, and having confounded the enemy at the first sight, led his army across the river in safety; and finding corn and cattle in the fields, after refreshing his army with them, he determined to march into the country of the Senones.

What bridges? It makes it sound like Caesar thought it was necessary to make bridges there and he had already started them. I don't think he did think it necessary. Nor did he make, nor had he started to make, pontoon bridges in that place. I think this is a hypothetical projection: if he would necessarily be at risk in constructing pontoon bridges there, then they should not be built, but something else done instead and as quickly as possible.

"When Caesar learned these things he figured that, if constructing bridges had to [necessarily] put him in peril, he should [instead] hurry to engage before the forces gathered there were greater. For he was thinking that, should he in a change of plan alter his route into the province, it wasn't something that fear would make someone do necessarily, when the shame and indignity of it, Mount Cevennes opposite, and the bad roads were preventing him, and especially because he was greatly fearing for Labienus who had been detached and for those legions he had sent along with him."